Recently we have found that mitogen- or antigen-stimulated T-lymphocytes produce a T-cell growth factor (TCGF) that allows for the maintenance of cytotoxic T-cells in continuous exponential proliferation. The TCGF is not species-specific and is consumed by proliferating T-cells. The proposed research is concerned with the characterization of TCGF, the initiation and functional characterization of cytotoxic, helper, and suppressor T-cell lines and the production of functional T-cell hybrids. The cellular origin, immunochemical nature, and in vitro, as well as in vivo, biologic activity of TCGF will be determined. Cytotoxic, as well as helper and suppressor, T-cell lines will be cloned and characterized as to their phenotypic and functional characteristics. Unifunctional continuous T-cell lines will then be used to produce antisera to differentiation antigens and to create functional T-cell hybrids.